The present invention relates to magnetic flow meters, and in particular, to circuits for converting sensed flow potential to an output.
In pulsed-DC electromagnetic flow meters, a transmitter provides a low frequency (typically 3-40 Hz) square wave current to coils on a flow tube to produce a square wave magnetic field in conductive fluid flowing through the flow tube. The magnetic field generates a square wave potential having an amplitude proportional to the flow according to Faraday's Law at electrodes on the flow tube. The potential is coupled through a cable back to the transmitter. The frequency of the square wave excitation is selected high enough to avoid interference from low frequency noise present in the flowing fluid, but low enough so that capacitive loading of the cable and inductive effects from the coils do not substantially distort the amplitude of the square wave potential. The flow transmitter converts the amplitude of the square wave potential to an output representing flow.
In the past a variety of conversion circuits have been used. Typically, the square wave potential is demodulated, providing a DC potential which, in turn, is presented to a voltage controlled oscillator (VCO) which provides a VCO frequency indicative of flow. Since a demodulated DC potential is presented to the VCO for conversion to a frequency, conversion accuracy depends on the zero point stability of the VCO. Maintaining the accuracy of the VCO at a desired level (0.1% of reading over a 50:1 range of flow, for example) over a long period of time with changes of temperature has been a problem. An arrangement is thus needed which does not require a high accuracy VCO for conversion.
Various circuits have also been used to convert the VCO frequency to an electrically isolated, adjustably scaled 4-20 mA analog output. Typically, the VCO frequency output is coupled to a divider which is adjustable to provide scaling. The output of the divider is fed through an optical coupler to a frequency to current converter circuit which converts the scaled frequency to a 4-20 mA output current. There is a desire to also provide a scaling adjustment with a wide range of adjustment, while providing a smooth, accurate conversion to a 4-20 mA output current from the scaled frequency.